//随机生成7种方块中的一种   
function getSquareInstance() {
    let num = parseInt(Math.random() * 7);
    // let str = "new " + "Square" + num +"()";
    // return eval(str);
    if (num === 0) {
        return new Square0();
    } else if (num === 1) {
        return new Square1();
    } else if (num === 2) {
        return new Square2();
    } else if (num === 3) {
        return new Square3();
    } else if (num === 4) {
        return new Square4();
    } else if (num === 5) {
        return new Square5();
    } else if (num === 6) {
        return new Square6();
    }
}